The role of hydrologic variability in the delivery of dissolved organic carbon and nitrogen to surface water

Hydrologic connectivity expands and contracts within watersheds as moisture conditions change, with implications for in-stream water quality. Dissolved organic carbon (DOC) concentration has been shown to increase in streams during high flow events, presumably as sources of DOC in side-channels, riparian soils, and/or uplands are mobilized and transported to surface water. Dissolved organic nitrogen (DON) has been less well studied, but behavior of DON is often assumed to mirror that of DOC. However, because DOC- and DON-rich pools of organic matter may be mobilized at different times, and because DOC and DON may undergo unbalanced abiotic and biotic processing along flowpaths, there is reason to suspect that DOC and DON respond divergently to high flow events. We use a meta-analysis to address the following questions: 1) To what extent do high flow events affect in-stream DON concentration? And 2) To what extent are DOC and DON concentrations decoupled during these events? Across 47 systems that included 78 high flow events, flow-weighted mean DON concentration increased, on average, 1.58-fold from baseflow to high flow, with a maximum increase of 9.5-fold. DOC and DON exhibited a complex relationship, with DOC and DON concentrations peaking at different times in over half of the events, and molar DOC:DON ratios varying, on average, 4-fold during high flow. An intensive field investigation of a tile-drained agricultural system in eastern Washington indicates that DOC and DON concentrations increase in tile drain discharge during winter runoff relative to baseflow, consistent with the mobilization of novel sources of DOM. However, DOM characteristics, including DOC and DON concentrations, DOC:DON, and fluorescence indices, of tile drain discharge can vary widely from potential sources identified in the watershed (ground water, soil water). For example, during a base flow period, DOC:DON was 41 in ground water, 24 in water-extractable soil organic matter, but only 1.6 in tile drain water. Additionally, the freshness index increased along a down-slope gradient in groundwater wells to the tile drain. These results suggest an important role for additional DOM sources (beyond groundwater and soil water), or organic matter processing during transport from soils to stream via tile drainage.